1. Field of the Invention
The present invention relates generally to film heat sealing machines used for closing and sealing bags, such as polyethylene bags and other containers for merchandise, under heat and pressure. More particularly, the invention relates to controlling the gap and thereby the pressure exerted in the sealing process between pairs of opposed heat transfer elements located on opposite sides of the path of the continuous traveling band which conveys films to be sealed through such machines.
2. Description of the Related Art
Sealing machines of the class to which the present invention generally pertains employ a pair of continuous bands, made of steel, fiberglass or other similar materials, which are arranged to travel in the same direction in confronting parallel relation with each other along a sealing run. The film laminae or layers of plastic material to be sealed together travel sequentially along the sealing run between the bands and the seal is made by simultaneously applying heat and pressure to the laminae as it travels between the bands along the sealing path. The material to be sealed is carried between a pair of continuous belts or chains also arranged to travel in the same direction in confronting parallel relation and at the speed of the sealing bands.
At least one pair of bars or platens flanking the continuous bands apply heat and pressure to seal the laminae through the bands. These heater bars are normally mounted on the frame of the machine and have generally been stationary with respect to the frame of the machine, so that they remain close to the traveling bands but allowance is made for the thickness of the film material to be sealed which travels therebetween. Ample room has had to be supplied so that the film material would not produce jamming of the machine in the event that wrinkles or multiple thickness were encountered during high speed operation.
More recently, the heater bars have been mounted on springs so that the entire heater bars are movable inwardly and outwardly a certain amount and apply a constant pressure against the moving bands and thus the film material being heat sealed so that the sealing is effected under spring loaded compression pressure. Allowance may then be made for the heater bars to separate a certain amount from each other and allows bands to separate or pivot slightly when wrinkled portions of film laminae or other multiple thicknesses pass through the machine during sealing. One such system incorporating spring-loaded heat transfer elements which continuously bear inward against the bands with a pressure derived from springs is shown in U.S. Pat. No. 4,016,026 to John J. Grevich and Stanley D. Denker, a co-inventor in the present invention. That device confers an amount of self-adjustment to the system which, in turn, does show some improvement to its ability to deal with varying thicknesses of material traveling between the bands.
While a great deal of progress has been made in the direction of solving the problems with regard to improving the consistency of machines for heat sealing film material, particularly where there is a possibility of variances in the thickness of the film being sealed, even the previous so-called self-adjusting systems lack the desired flexibility as to what materials can be processed. They also lack sufficient sensitivity to thickness changes desireable for present day high speed operation. Thus, improved quality of seals produced together with freedom from jamming are still sought after goals with such machines.